Somebody else will need to help you, but I think this is them:

https://www.scan.co.uk/products/kioxia-19tb-cm7-r-u3-sie-pcie-gen5-1x4-2x2-u3-15mm-read-intensive-1dwpd-enterprise-ssd

For it to work at all, pretty sure it needs to appear in BIOS somewhere. 

Finally, I don't think those motherboard-ports you see are traditional SATA ones. NVMe is something else (much faster) and is usually PCIe based. U.3 is like for large capacity NVMe-SSDs that are too large to fit on a conventional 2280 form-factor card.

Get the computer's PDF manual.

Hello, those are U.3 enterprise nvme ssd. You don't use them with the normal sata port of the mb... you either install them in pci-e slots ( one slot - one drive ), or you need the nvme flexbay option installed . But I should point out that the technical guidebook for the 5860 mentions those nvme flexbays to be M.2 compatible . It never mentions to also support U.3 (or U.2) ( while in the Precision 5820/7820/7920 it's expressly written M.2 and U.2 support )

@mazzinia_​ I do have the flexbay installed and they are connected to the SATA 0 and SATA 1. If I insert a SATA SSD drive I can see the drive shown in the BIOS but if I insert those NVMe drives nothing showing.

@danzi100​ Because of what I wrote. You have the flexbay for SATA , not the Flexbay for NVMe , installed. 
Your computer has one of these configurations : 
2 x sata flexbays
1 x sata , 1 x nvme flexbay
2 x nvme flexbays

The sata flexbay is able to see only sata drives. The U.3 enterprise drive is not a sata unit. It's an U.3 nvme enterprise ssd.

If you install one of those U.3 units on an adapter that installs directly in a pcie slot , it should be visible.
If you change the flexbay with the nvme one, it may work but I cannot offer guarantees because Dell never mentions supporting U.2/U.3 on the nvme flexbay offered for that computer ( I think ), so it would be down to testing .

I have an U.2 enterprise unit in the nvme flexbay of my Precision 7820, but in the 7820 manual is explicitly written that the flexbay nvme slot on it supports both nvme and U.2 units.

To add to mazzinia comment, in other words, to make KIOXIA U.3 drives appear in your Dell Precision 5860, you need to upgrade your flexbay backplane with tri-mode storage host. 

Note that while the U.3 drives will be backward compatible and working when connecting to U.2 slots, the U.2 drives will not be support when connecting to U.3 host.

@Chino de Oro​ The exciting front and inside the case flexbay take SSD drives and NVMe drive you can access the SSD drive not the NVMe. Do you have a link for the flexbay backplane with tri-mode storage host

While I do not have Dell part number, the 5860 system configuration does have selection for tri-mode setup by selecting MegaRAID SAS 9660-16i.  Dell would then install proper backplane which connecting to storage controller that can support NVMe including form factor of U2/U3.

If your current flexbay already have NVMe SSD carrier JX33G, or 66XHV, or DPWC700, you can replace it with SATA HDD bracket with 2.5" adapter to test your U.3 drive.